Case-hardened steel product and process for its manufacture

ABSTRACT

The product comprises a tough core and at least one hard, carburized surface layer, the core containing 0.01-0.04% C, 2.26.0% Mn, 0.5-2.0% Si, up to 0.2% acid-soluble Al, 0.01-0.5% Nb and/or V, with Nb being replaceable by Ta, up to 0.03% N, up to 0.2% Zr, the balance being iron and impurities, the surface layer having a higher carbon content than the core but otherwise the same composition. The product is manufactured by shaping a blank of the aforesaid core composition, the shaping operation being followed by surface carburization and hardening.

O Unlted States Patent 1151 3,661,656 Jarleborg 1 1 May 9, 1972 [54]CASE-HARDENED STEEL PRODUCT [56] References Cited AND PROCESS FOR ITSUNITED STATES PATENTS MANUFACTURE 1,990,647 2/1935 Harris.... ..148/31.5[721 Invent Jarlebmg, Hedkamvagen 2 219 780 10/1940 Jominy ..14s/3 1.5

Sweden [73] Assignee: Fagersta Bruks Aktiebolag, Fagersta, U'Examiner-Richard Dean Sweden Att0rneyH0lman & Stem [22] Filed: June 3,1969 57 ABSTRACT [21] PP N 829,949 The product comprises a tough coreand at least one hard, carburized surface layer, the core containingOBI-0.04% C, Foreign Appncation priority Data 2.2-6.070 Mn, (LS-2.0% Si,up KC? 0.2% acid-soluble A1, 0.010.5% Nb and/or V, w1th Nb bemgreplaceable by Ta, up June 14, 1968 Sweden ..8093/68 to 003% N, up to02% Zr, the balance being iron and imPurities, the surface layer havinga higher carbon content than the 52 us. (:1 ..148/12.1, 148 124,148/165, core but otherwise the same composition, The product is 148/19,148/315, 148/36, 148/39, 148/144 manufactured by shaping a blank of theaforesaid core com- [51] Int. Cl. ..C21d 1/78 position, the Shapingoperation being f ll d by Surface 58 Field of Search ..148/16.5, 19,134, 137, 143, b i ti and hardening m/mi/ CASE-HARDENED STEEL PRODUCTAND PROCESS FOR ITS MANUFACTURE This invention is concerned with the useof an alloyed steel for the manufacture of case-hardened products orarticlessuch as rods, bars, tubes, gear wheels, detachable rock drillbits and the like-which must have a hard, wear-resistant surface and atough core portion. The products are made of a manganese steel having alow content of carbon but having a surface of high carbon contentproducedby carburization. The essential feature of the products orarticles according to the invention consists in that they are made of akilled steel comprising, in addition to iron and ordinary impurities,

C 0.0l-0.04 preferably 0.01- .03 Mn 2.2-6.0 preferably 2.5-5.0 Si0.5-2.0 preferably 0.7-1.5 a1. (acid-soluble) up to 0.2 preferably0.0l-0.1 Nb and/or V 0.0l-0.5 preferably 0.02-0.2

but Nb being replaceable wholly or partially by Ta N up to 0.03preferably 0.005-0.02 T Zr up to 0.2

and having a hard surface which is rich in carbon and is obtained inthat the products or articles after having been shaped are subjectedwholly or partially to carburization of'their surface portion, followedby hardening. Thus, except for its carbon content, the carburizedsurface layer has the same composition as the core portion.

The non-carburized steel has a very fine grain structure andextraordinarily good strength properties. By variation of the manganesecontent of the steelthe yield point and strength may be made to varywithin wide limits, according to needs. The yield point will be withinthe range of from 60 to 100 kg/sq.mm. The material is very tough, andits impact transition temperature is below -40 C even at maximumstrength. As stated above, the steel is to contain 2.2-6.0 percent ofmanganese, preferably 2.5-5.0 percent. For some purposes a manganesecontent of 3.0-5.0 percent is preferred.

The silicon content of the steel is to be within the range of from 0.5to 2.0 percent. With a Si content below the 0.5 percent limit, thecarburization would proceed slowly or would be non-uniform. Thesiliconcontent-which is preferably within the range of 0.7-1.5 percentis of importance for the carburization treatment because siliconincreases the activity of carbon so that the product can thus be given auniformly carburized surface layer within a relatively short time.Moreover, residual austenite which is normally obtained in carburizingoperations, especially if the manganese content is high, will disappearwhen silicon is present within the ranges stated above.

Furthermore the steel is to contain 0.0l-0.5 percent, preferably0.02-0.2 percent of niobium (tantalum) and/or vanadium. Normally itshould contain max. 0.3 percent niobium (tantalum) for reasons ofsolubility, and it should preferably not contain more than 0.3 percentvanadium, chiefly for reasons of economy.

The steel lends itself extremely well to shaping operations and caneasily be subjected to cutting tool working operations. The diagram ofthe accompanying drawing illustrates the machinability of a steel No.2736 in a milling test, as compared to a conventional steel No. 2541subjected to the same milling test. Steel No. 2736 contains 0.032 C,1.15 Si, 3.85 Mn, 0.070 AL, 0.13 Nb and 0.015 N and has beenheat-treated to a yield point of 76 kg/sq.mm, a tensile strength of 93kg/sq.mm and a Brinell hardness of 330. The conventional steel No. 2541contains about the following proportions ofC, Si, Mn, Cr, Ni and Mo: C0.35 Si 0.25 Mn 0.7%; Cr 1.4 Ni 1.4 M 0.2 and this steel has beenheat-treated to a Brinell hardness of 270. lnthe diagram, the volume ofcuttings in cubic centimeters (ordinate) is plotted against the cuttingrate in meters per minute (abscissa), the milling having been performedwith a high-speed steel (cutting depth 1 mm, feed 0.1 mm, criterionVB,,,,, 0.7 mm).

Shaping of the steel may be effected in many different ways. A blank maybe produced by casting, rolling, forging etc. The blank is then giventhe desired shape by machining or hot or cold pressing.

The object or body thus obtained is then subjected to surfacecarburization in those portions where surface hardness is desired.interior as well as exterior portions of the body configuration may becarburized. The carburizing operation is carried out in some known perse manner such as for instance by powder-carburization,cyanide-carburization or gas-carburization. The thickness of .thecarburized surface layer may vary from 0.1 to about 2.5 mm according tothe dimensions of the product. Normally carburizing depths near or atthe lower end of this range are chosen for articles of small dimensionswhereas carburizing depths in the vicinity of the upper end of saidrange are chosen for machine parts of large dimensions.

The carbon content in the carburized portion will amount to 0.5-1.5percent, preferably 0.7-1.2 percent. By way of example, it may bementioned that in one case the treatment of the steel product waseffected at 900 C for 6 hours, to give a carburized layer which had athickness of 1 mm and a carbon content of about 0.9 percent.

In the simplest case, hardening is effected immediately after thecarburization and from a temperature slightly lower than that at whichcarburization was carried out. Alternatively, the product may be allowedto cool after the carburization treatment, whereupon it is re-heated tothe hardening-temperature and hardened. The hardening may be effected inwater, oil, air, salt bath or the like. The treatment of the productusually comprises also a low-temperature tempering, as a final step.After completion of the heat treatment the product generally has asurface hardness of 55- 65 Re.

One example of a product or article according to this invention is adetachable bit for a rock drill. When such a bit is to be manufactured,a blank is shaped to the desired bit shape in the manner stated above,whereupon the surface layer is carburized on the interior and/orexterior faces. Then, hard metal pieces are soldered onto the bit, thisstep involving a heat treatment as required. In a hard solderingoperation, the bit is as a rule heated partially to a temperature ofabout l,000-1 ,200 C. Also, a convenient way of proceeding consists infirst heating the entire bit to a lower temperature such as for instance800-900 C and then continuing to heat the foremost portion of the bitbody-where grooves, recesses or holes have been provided for the hardmetal pieces-up to the soldering temperature. Other methods of heattreatment may also be employed.

Another example of a'product or article according to this invention is agear-wheel which has been given its desired shape in the manner statedabove. When the wheel body has been subjected to a machining or otherfinishing operation the cogs are carburized in their surface portions,whereupon the gearwheel is heat-treated by hardening and tempering to adesired degree of hardness.

The products or articles of this invention have great advantages asregards their properties and the ease with which they can bemanufactured on an industrial scale. In particular, the following pointsshould be noted:

1. The starting material is a steel that lends itself very easily toallkinds of working operations, thus reducing the costs of operation toa minimum.

2. Due to its carburized surface, the product shows high resistance towear, and high fatigue strength.

3. The core portion of the material retains its toughness also after thecarburization has been effected.

4. Due to its hard surface layer and tough core, the product has a highdegree of resistance to shocks and impacts, the material providing ashock absorbing effect.

5. Due to the toughness of the starting material, the extent to whichheat treatments (such as performed in soldering operations) will causechanges in the dimensions of the product is minimized; this in turnresults in a more favorable stress distribution in the soldering joint.

consisting of:

0.0l-0.04 1b, Mn 2.2-6.0 96; Si 0.5-2.0 '5; Al (acid-soluble) up to 0.2

at least one element selected from the group v consisting of Nb or V inan amount of from and Nb being replaceable wholly or partially by T N upto 0.03 56; and Zr up to 0.2 k;

the balance being iron and ordinary impurities; and at least one hard,carburized surface layer having a higher'carbon content than said corebut having otherwise, except for said surface layer carbon content, thesame composition as said core. 2. The steel product according to claim 1being of detachable rock drill bit having at least one carburizedsurface layer.

3. The steel product according 'to claim 1 being a gearwheel having cogswherein the surface layers are carburized.

4. A process for producing a case-hardened steel product made of akilled, manganese-alloyed steel blank consisting of:

C 0.01-0.04 Mn 2.2-6.0 Si 0.5-2.0 Al (acid-soluble) up to 0.2 Kw;

at least ,one element selected from the group consisting of Nb or V inan amount of from the balance being iron and ordinary impurities;

comprising shaping the steel blank to the desired configuration of theproduct; carburizing at least one of the surface layers of the shapedsteel blank; and then hardening to form the desired steel product. i

5. The process according to claim 4 wherein said hardening is followedby-a tempering step.

6. The process according to claim 4 wherein said shaping step is amachining operation.

7. The process according to claim 4 wherein said shaping step is apressing operation.

8. The steel product according to claim 1 wherein the compositionconsists of C 0.01-0.03 i. Mn 2.5-5.0 5; Si 0l7-l.5 i; Al (acid-soluble)0.0l-0.l is;

at least one element selected from the group consisting of Nb or V in anamount of from'0.02-0.2 and Nb being wholly or partially replaceable byTa;

N 0.005-002 and Zr up to 0.2 the balance being iron and ordinary impuri-C 0.01-0.03 Mn 2.5-5.0 9%; Si 0.7-1.5 Al (acid-soluble) 0.0l0.l

at least one element selected from the group consisting of Nb or V in anamount of from 0.02-0.2 and Nb being wholly or partially replaceable byTa;

N 0.005-0.02 and Zr up to 0.2 the balance being iron and ordinaryimpurities. 12. The process according to claim 11 wherein said hardeningis followed by a tempering step.

* l l l

2. The steel product according to claim 1 being of detachable rock drillbit having at least one carburized surface layer.
 3. The steel productaccording to claim 1 being a gear-wheel having cogs wherein the surfacelayers are carburized.
 4. A process for producing a case-hardened steelproduct made of a killed, manganese-alloyed steel blank consisting of: C0.01-0.04 %; Mn 2.2-6.0 %; Si 0.5-2.0 %; Al (acid-soluble) up to 0.2 %;at least one element selected from the group consisting of Nb or V in anamount of from 0.01-0.5 %; and Nb being replaceable wholly or partiallyby Ta; N up to 0.03 %; and Zr up to 0.2 %; the balance being iron andordinary impurities; comprising shaping the steel blank to the desiredconfiguration of the product; carburizing at least onE of the surfacelayers of the shaped steel blank; and then hardening to form the desiredsteel product.
 5. The process according to claim 4 wherein saidhardening is followed by a tempering step.
 6. The process according toclaim 4 wherein said shaping step is a machining operation.
 7. Theprocess according to claim 4 wherein said shaping step is a pressingoperation.
 8. The steel product according to claim 1 wherein thecomposition consists of C 0.01-0.03 %; Mn 2.5-5.0 %; Si 0.7-1.5 %; A1(acid-soluble) 0.01-0.1 %; at least one element selected from the groupconsisting of Nb or V in an amount of from 0.02-0.2 % and Nb beingwholly or partially replaceable by Ta; N 0.005-0.02 %; and Zr up to 0.2%; the balance being iron and ordinary impurities.
 9. The steel productaccording to claim 8 being a detachable rock drill bit having at leastone carburized surface layer.
 10. The steel product according to claim 8being a gear-wheel having cogs wherein the surface layers arecarburized.
 11. The process according to claim 4 wherein the compositionconsists of: C 0.01-0.03 %; Mn 2.5-5.0 %; Si 0.7-1.5 %; A1(acid-soluble) 0.01-0.1 %; at least one element selected from the groupconsisting of Nb or V in an amount of from 0.02-0.2 % and Nb beingwholly or partially replaceable by Ta; N 0.005-0.02 %; and Zr up to 0.2%; the balance being iron and ordinary impurities.
 12. The processaccording to claim 11 wherein said hardening is followed by a temperingstep.